If you want to see a killer image that uses snow on mountains, check out Bruno's Mt. Rainer post:

http://www.renderosity.com/mod/gallery/index.php?image_id=1326509

For closeup snow, I created a procedural snow mat that looks reasonably convincing (not as good as Bruno's however):

http://www.renderosity.com/mod/gallery/index.php?image_id=1329787